Crowdfunding may supply the rooftop solar projects with $5 billion of investment within five years, more than 50 times the amount raised to date.

That would represent more than a quarter of all annual investment in that segment of the solar industry, said Tim Newell, vice president of financial products for San Mateo-based SolarCity, which is the biggest U.S. solar power provider by market value.

A growing number of rooftop solar developers are soliciting funds directly from retail investors, often through websites that tap a large number of small contributions. This so-called crowdfunding model has attracted almost $100 million in the U.S. to date, Newell said. It offers one of the few ways for individuals to back renewable energy projects, which give steady, long-term returns from selling electricity.

"This is 'crowd' with a capital 'C,'" Newell said Tuesday in an interview at the Bloomberg New Energy Finance conference in New York. "Crowd isn't a niche thing. It's everything but that top handful of institutions."

Money that otherwise would have been poured into the municipal or corporate bond markets or sat idle in investors' bank accounts will find its way into supporting solar systems, he said.

"You'll see a surprising amount of assets move through this channel in the next few years," Newell said. "And we don't say that just because of misguided optimism. We're saying it because we take a hard look at it and spend a lot of time talking to investors."

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U.S. homeowners installed 792 megawatts of solar power last year, up 60 percent from 2012, according to data compiled by the Solar Energy Industries Association.